Data storage systems are arrangements of hardware and software that include one or more storage processors coupled to arrays of non-volatile storage devices. The storage processors service storage requests, arriving from host machines (“hosts”), which specify files or other data elements to be written, read, created, or deleted, for example. Software running on the storage processors manages incoming storage requests and performs various data processing tasks to organize and secure the data elements stored on the non-volatile storage devices.
Some data storage systems employ storage tiering. Such systems include multiple tiers of non-volatile storage with each tier providing a different quality of service. For example, a system may include a first tier (Tier 1) for SSDs (solid state drives), a second tier (Tier 2) for SAS (Serial-Attached SCSI) drives, and a third tier (Tier 3) for SATA (Serial Advanced Technology Attachment) drives, for example. The system manages placement of data on the different storage tiers to make the best use of disk drives overall. For example, frequently accessed data may be placed on Tier 1 storage, less frequently accessed data may be placed on Tier 2 storage, and seldom accessed data may be placed on Tier 3 storage.
Some data storage systems perform storage tiering automatically. These systems monitor the activity of storage elements and move data between storage tiers to best utilize available resources and promote efficiency. For example, a set of data stored on SATA drives may be moved to SSDs in response to the system detecting that the set of data is frequently accessed. Likewise, a set of data stored on SSDs or SAS drives may be moved to SATA drives if the system detects that the data is rarely accessed. Automatic storage tiering (also called AST or auto-tiering) thus helps to ensure that disk drives of different types are put to their most effective use.